Modern satellite communication systems provide a robust and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems for carrying data traffic such as Internet traffic. A typical satellite Internet system comprises subscriber terminals, a satellite, a ground station, and connectivity to the internet. Communication in such a system occurs along two paths: 1) a forward path comprising an uplink from a subscriber terminal to the satellite to a downlink to the ground station to the internet; and 2) a return path comprising a path from the internet to the ground station to an uplink to the satellite to a downlink to the subscriber terminal.
Adaptive coding and modulation (ACM) is a technique used in some satellite systems to adapt the modulation and/or forward error correction (FEC) coding of the transmitted waveform to the conditions of the link as appropriate for each receiving terminal. Per-terminal link conditions might be affected by substantially static factors such as antenna pointing accuracy of a transmitting or receiving terminal, as well as by dynamic factors such as changing weather attenuation affecting uplink or downlink propagation. The objective of ACM is to meet a target receive signal quality at the destination of the communication path (e.g., bit error rate, packet loss rate, or other metric) while maximizing link capacity (e.g., bits/symbol or bits/Hertz).
ACM might be used to optimize a point-to-point communication link, such as from a terminal transmitting an uplink carrier, through a transponded, bent-pipe, satellite, to a terminal receiving the transponded carrier on the downlink. ACM may also be used to optimize point-to-multipoint links, such as an uplink carrier transmitted through a bent pipe satellite to a set of receivers within the same downlink spot beam, onto which carrier are multiplexed data packets each destined to one or more of the several receiving terminals.
To date, ACM techniques have not been applied in regenerative satellite systems. In a regenerative satellite system, the satellite itself has digital processing hardware. This provides several advantages. First, the regenerative satellite demodulates, decodes, encodes, and remodulates the digital uplink signal to form a downlink signal. This provides separation of the uplink from downlink by regenerating the digital signal and can save in the link budget's signal strength. Second, this digital on-board processing permits the satellite to take the data packets it receives on the uplink and route or switch it to particular downlink locations. Additionally, satellite demodulators on the uplink may be assigned to particular uplink spotbeams to listen to assigned uplink frequencies, i.e., uplink sub-bands. The satellite's uplink antennas and demodulators are able to separate and differentiate signals from satellite terminals transmitting from these uplink spot beams. This allows reuse of a same frequency in different uplinks, providing a major increase in frequency efficiency.